Tellurite glasses having compositions that provide increased bandwidth of the emission spectra of rare earth dopants in the glasses and enhanced thermal stability of the glasses, and optical components that are composed of these glasses and that are used in telecommunication systems.
Optical components, in particular components employing optical fibers doped with rare earth metal ions, are widely used in telecommunication systems. A major application is in signal amplifiers which employ fluorescent ion emission for amplification of a signal. The ion emission occurs within the same operating wavelength region as the signals. Pump energy excites the rare earth metal ion causing it to fluoresce and thereby provide optical gain.
Glasses, doped with a rare earth metal ion and pumped with appropriate energy, exhibit a characteristic, fluorescence intensity peak. The evolution of telecommunication systems has created a need for a rare-earth-doped, amplifier material having the broadest possible emission spectrum in the wavelength region of interest. It is a purpose of the present invention to meet this need.
The bandwidth of a fluorescent intensity curve is, rather arbitrarily, taken as the full width half maximum (FWHM) of the curve in nanometers wavelength. This value is the lateral width of the curve at one half the maximum intensity, that is, at one half the vertical height of the peak of the curve. Unfortunately, many glasses, that exhibit a fluorescence in an appropriate region, exhibit a rather narrow bandwidth. It is a further purpose of the invention to provide a family of glasses that exhibit a relatively broad bandwidth.
It is well known that glasses doped with erbium can be caused to emit fluorescence in the 1520-1560 nm. region. This enables a signal operating in this wavelength range to be amplified. The significance of the 1550 nm, wavelength region in optical communication has led to extensive studies regarding the behavior of erbium as a rare earth metal dopant in glasses. It has also led to the study of a variety of glasses as the host for the erbium ion.
The low phonon energy of tellurite glasses can lead to long emission lifetimes for certain pump wavelengths. As an example, erbium in tellurite glasses exhibits long tau-32 (980 nm. emission) values relative to silicates. The long emission lifetimes at this practical pump wavelength can reduce the efficiency of an amplifier or laser because of insufficient population at the upper laser level and excited state absorption. A practical 980 nm. pumping scheme can be obtained by co-doping glasses with low mass components having phonon overtones that are resonant with the energy difference between the 980 and 1530 nm. levels. Such components include H2O, B2O3, P2O5.
For certain applications, a long, erbium, tau-32 emission value, that is, a long erbium, emission lifetime at 980 nm, is desirable. These include long-band amplifiers, where pumping directly into the ground-state absorption does not impact noise figure; also tilt-free amplifiers, in which 980 nm. and 1480 nm. pump lasers are combined. The latter dynamically adjusts the gain without affecting the shape of the gain spectrum.
It is also known that glasses with moderately low maximum phonon energies, when doped with thulium, can display fluorescence in the vicinity of 1450 nm. Although this wavelength lies outside of the currently used telecommunications band, it still lies within the transparency window of most commercial optical fiber. The ever-increasing demand for useful bandwidth will create a need for additional amplifier devices that operate over the remaining portions of this window that are not covered by erbium.
It is known that, as the concentration of a rare earth metal ion, such as erbium, is increased, the optical gain increases up to a certain point. Beyond this point, the fluorescent signal is quenched, and the optical gain decreases. This phenomenon is considered to result from the dopant, rare earth metal ions interacting with each other in a manner commonly referred to as clustering. It is another purpose of the invention to provide a family of glasses which is readily capable of dissolving erbium ions, and which exhibits a broad bandwidth indicating that clustering is inhibited.
It has been reported that certain tellurite glasses, doped with erbium ions, provide a very broad, erbium emission band in the 1540 nm. region of the spectrum. Glass compositions were not there reported, but other sources indicate that the glasses are alkali-alkaline earth-zinc-tellurite glasses.
Tellurite glassesxe2x80x94oxide glasses based on TeO2xe2x80x94are unusual in that the basic, network-forming, Texe2x80x94O polyhedra can vary from trigonal TeO3 pyramids to distorted TeO4 groups. The latter can be regarded as trigonal bipyramids lacking an equatorial oxygen. Raman spectroscopy has shown that TeO4 groups dominate the structure of these glasses at high TeO2 content, but are progressively replaced by TeO3 groups with rising concentration of other oxides.
The diversity of TeOx species was thought to yield a greater diversity of structural sites for the incorporation of dopant ions, such as erbium ions. This would avoid the ions clustering and becoming ineffective for fluorescent emission and consequent amplification. This diversity of dopant sites should give rise to broadened, emission spectra. It is, then, a basic purpose of the present invention to provide a family of tellurite glasses in which compounds of rare earth metals, such as erbium, are readily soluble due to a diversity of compositional sites with which they can associate.
Both TeOx species are highly asymmetric due to the stereochemical effect of the lone 5s electron pair on the central Te4+ ion. On account of the latter, as well as the high atomic number of Te, these glasses are perhaps best known for their high refractive index that ranges from about 1.9 to 2.2. Another consequence of the stereochemically active, lone electron pair on Te4+ is the high optical nonlinearity of these materials, reportedly on the order of 10xe2x88x9213 esu at 1060 nm. This is roughly forty times that of fused silica.
The invention resides, in part, in a family of tellurite glasses having compositions that consist essentially of, as calculated in cation percent, 65-97% TeO2, and at least one additional oxide of an element selected from the group consisting of Ta, Nb, W, Ti, La, Zr, Hf, Y, Gd, Lu, Sc, Al, Ga and mixtures.
The invention further resides in an optical component for a telecommunication system that is composed, in part at least, of a tellurite glass that has a high thermal stability (Txxe2x88x92T9), that readily dissolves rare earth metal oxides and that has a composition consisting essentially of, as calculated in cation percent, 65-97% TeO2, at least one additional oxide of an element having a valence greater than two, the element being selected from the group consisting of Ta, Nb, W, Ti, La, Zr, Hf, Y, Gd, Lu, Sc, Al and Ga and mixtures, and 0.005-10% of a lanthanide (Ln) oxide.